The present invention relates to process for measuring elongation distribution of an optical fiber cable.
An optical fiber cable has the nature that the bandwidth of the same is quite wide and the transmission loss is small, therefore, that cable is promising for transmission medium for a long distance digital transmission cable. However, the optical fiber cable has the disadvantage that it is mechanically weak, in particular, a submarine optical fiber cable elongates 0.3-0.4% when installed, and also elongates 0.6-0.7% when it is picked up on a deck of a boat for repairing. The characteristics of an optical fiber cable depends upon the elongation of the same, therefore, the measurement of the elongation of an optical fiber cable is essential to keep the high reliability of an optical fiber cable.
A prior method for measuring elongation of an optical fiber cable is first described in accordance with FIG. 1, in which an optical pulse 2 is applied to an input end 11 of an optical fiber cable to be measured, and an output pulse is derived at the other end 12 of the cable, then, the delay time (.tau.) by the transmission of the optical pulse provides the elongation .DELTA.L of the cable. That value, elongation .DELTA.L, is calculated by the known equation (1) as follows. EQU (.tau.)=(N/c)L+(N'/c).DELTA.L (1)
In the equation (1), N is the group refraction index of a cable to be measured, N' is the equivalent group refractive index which is defined by the optical elastic effect by the cable 1, c is the light velocity in free space, L is the original length of the cable. Other prior system for measuring elongation of an optical fiber cable is phase method, or interference method, which uses the same theoritical principle as that of FIG. 1.
However, a prior method for measuring elongation of an optical fiber cable has the disadvantage that it can measure only the total elongation of the cable, but it can not provide an information where and how a cable is elongated. It should be noted that the characteristics of the optical fiber cable are not determined by an average elongation of the cable, but they depends upon the history of the elongation at various points of the cable.
Therefore, it is essential to measure the distribution of the elongation of an optical fiber cable for the study and the design of an optical fiber cable system.